Control & Testing

This section shows how to setup the produced combustion-driven actuator (CDA) for testing. This involves connecting the actuator to fuel lines and the necessary control hardware. As mentioned earlier, only a few extra parts are actually needed. A detailed Bill of Materials listing all parts and materials needed to build the combustion-driven actuator designed in this documentation can be found here.


To setup the CDA you have created, you will need three additional components:

  • Spark igniter with a spark gap of a few millimeters
    • The spark igniter is produced by skinning (stripping) two copper wires. They are put into different Teflon tubing for insulation purposes and filled with the same silicone mixture. This produces a simple but very robust igniter. The figure below further describes the manufacturing process of these spark leads.

Click here for larger image

  • Exhaust tube
    • The exhaust tube should have a smaller diameter than the injection nozzle. This guarantees a higher backpressure which will result in a higher combustion. Alternatively, valves could be used to actively control closing and opening times of the combustion chamber.
  • Injection nozzle
    • The injection nozzle should have a check valve. This valve closes upon a combustion event to prevent back pressure from the fuel lines which also increases expansion of the CDA. Such valves can be purchased from Parker / Legris.

Click here for larger image

The spark igniter is then connected to a spark transformer. This spark transformer will take a 5 V input and step it up significantly so you can get a reliable spark from your igniter. Code running on the programmable logic controller (PLC) can then close the relay at desired intervals to control the ignition sparks. We use PLCs from Siemens (Logo) in this documentation, but Arduinos (as used in many other guides in the toolkit) are an equally good option to control ignition. The PLC controls gas flow as well by addressing two mass flow controllers (MFC), purchased from Vögtlin. The protocol we used in this demonstration can be downloaded here.


To test the CDA, the following parameters were used:

  • Methane flow rate: 0.4 – 0.6 L/min
  • Air/Methane ratio: 10
  • Inlet pressure air/methane: 2 bar (required by the listed MFC’s)
  • Ignition frequency: 0.5 Hz

The PLC uses an analog signal in order to control the amount flowing through the MFCs. This signal varies within a range of 0 to 10 V, corresponding to an analog value of 0 to 1000. Therefore, one has to devide the maximum analog value (AV) by the maximum flow rate (FR) and multiply times the desired flow rate to convert the desired flow rate to an analog value for the PLC. An example:

Desired methane flow rate = 0.6 L/min
→max AV/max FR * desired FR
→1000/20*6 = 300

This value now can be entered at the designated position in the protocol (shown below).

The final result is a combustion-driven actuator, able to increase its volume five-fold after less than half a second. Only a few equipment parts are needed in order to operate such an actuator. The concepts presented here can be used in many ways. For instance, the volume increase can be used to power soft pumps, which is shown in the case study section.